NANO AT HOME: An Experiment That You Can Try
PLEASE NOTE: The Center for Nano- and Molecular Science and Technology (CNM) at The University of Texas at Austin (UT-Austin) cannot guarantee the accuracy or the safety of these activities. Some of these activities might pose safety hazards for young children, and all activities should be performed under the supervision of a responsible parent, teacher or adult. The CNM and UT-Austin do not assume any responsibility for these activities or their results. If you have questions, corrections, or comments please do not hesitate to contact the CNM.
You might have seen a demonstration of iron filings sprinkled on paper placed over a magnet. You may have noticed how the iron filings trace the path of the magnetic field around the magnet, looping from one magnetic pole to the other, like in the photo at right. It’s also possible to make iron filings very small and to suspend them in a liquid. The liquid’s ability to flow will then change when the liquid is brought near a magnet.
Rheology is the study of how matter flows. Some fluids, like water or oil, flow faster when more force is applied. Some fluids, like ketchup, flow faster when they are agitated by shaking. Some fluids, like Silly Putty®, actually become more firm and will shatter when forced to move too quickly. A magnetorheological fluid, often abbreviated MR fluid, is a fluid that changes the way it flows in the presence of a magnetic field. A typical MR fluid is made of very small solid particles that are suspended in a liquid and attracted to a magnet.
You can make your own MR fluid by following the simple steps below:
Small magnetic particles
Liquid such as vegetable oil
Stirring Stick (like a popsicle stick)
Disposable Cup, plate or pie tin
The magnet must have a bit of magnetic field strength, say, enough to lift a full soup can. Many ferrite ceramic magnets will work; “neodymium” magnets will likely work even better. CAUTION: Strong magnets can pinch flesh and also should never be swallowed.
Small magnetic particles
This includes iron or iron oxide powder, obtained either by grinding or filing a piece of iron or by purchasing iron powder from a teaching supply catalog. We like to use “black iron oxide” purchased from a pottery supplier. This powder is used to color ceramic glazes for pottery and is composed of magnetite (Fe3O4). Particles of this powder are shown in the image below. Notice that some of their features are less than 100 nm in size. CAUTION: Fine powders can be challenge to clean up! Covering your table or other work surface with newspaper before you begin is highly recommended.
The liquid should be “thicker” or more viscous than water. Vegetable oil or hand soap are some good choices. CAUTION: Choose a non-toxic liquid.
Mix approximately equal amounts of the magnetic powder and the liquid in a disposable cup (adding in a little more powder or a little more liquid can adjust the flow properties of the mixture). After mixing thoroughly, pour the resulting MR fluid into your disposable container and observe how well it flows. Then, place the magnet underneath the container and observe how the MR fluid flows less easily. The MR fluid becomes “thick” enough to support objects, almost like a solid. If you use a liquid that can harden over time, such as paint or glue, the spikes can stay standing even after the magnet is removed.
(A) Iron oxide mixed with motor oil MR fluid in the absence of a strong magnetic field. (B) When the MR fluid is exposed to a strong magnetic field it becomes much more rigid and is able to support a stick standing on end indefinitely. (C) Note the “spikes” in this MR fluid as the particles try to follow the magnetic field lines of the magnet under the container.
A small wooden stick was used to paint a mixture of white glue and magnetite powder as patterns of spikes onto paper.
What is happening?
The viscous carrier liquid helps the iron or iron oxide particles to stay suspended. This allows the particles to move about with minimal clumping when the MR fluid is stirred or poured. It also allows the particles to move in three dimensions when a magnet is brought near the mixture. In this way the particles can orient themselves with the magnetic field and peaks can form.
The smaller the particles are the more easily they stay suspended in the carrier liquid. For the finely ground iron oxide from the clay supplier, the particles will stay suspended for more than a week in motor oil. Finer particles also allow the fluid to flow more easily when no magnetic field is present and cause the fluid to stiffen up more completely when a magnetic field is present. When the MR fluid is exposed to a strong magnetic field the iron or iron oxide particles align with the magnetic field and form a network that traps the carrier fluid. This is sort of like a large group of people holding hands. It becomes very difficult for any one person to move very far. This causes the bulk fluid to behave more like a solid.
If you looked at an MR fluid under high magnification in the absence of a strong magnetic field it would look like the illustration on the left. There you can see individual iron oxide particles suspended in a viscous carrier fluid. If you brought a strong magnet near the MR fluid the individual iron oxide particles would align with the magnetic field and clump together like the image on the right.
Connection to the nanoscale
Making the particles very small, about 10 nanometers across, produces a ferrofluid. In a ferrofluid the particles are so small that they will never settle out of the carrier fluid. The particles do not adhere to each other because they have a special coating. So the ferrofluid can flow to form whatever shape the magnetic field causes it to be. Ferrofluids are true nanomaterials and are leading to exciting innovations in the field of active suspension systems and medical research.
On the left: ferrofluid flows and acts like any normal liquid in the absence of a strong magnetic field. On the right: ferrofluid on top of a strong magnet forms peaks which follow the magnetic field.